A fuel cell is an electrochemical energy conversion device that generally converts chemical energy into electric energy through oxidation and reduction of hydrogen and oxygen.
At an anode, oxidation occurs to split hydrogen into hydrogen ions and electrons. The hydrogen ions are moved to a cathode through an electrolyte, and the electrons are moved to the cathode through a circuit. At the cathode, reduction occurs to combine the hydrogen ions and the electrons with oxygen to produce water.
Since it is impractical to use only a single unit cell of the fuel cell due to its low voltage, a stack of several to several hundred unit cells is generally used in practice. Here, a separator is provided to ensure electrical connection between unit cells while separating a reaction gas in the stack of unit cells.
In past times, graphite-based separators were generally used. Recently, however, metal separators have been actively developed to overcome problems of the graphite-based separators such as high brittleness of graphite and high manufacturing costs.
The separator is required to have a sufficiently high electrical conductivity for ensuring electric connection between the respective unit cells. Further, since the interior of the fuel cell has a high concentration of hydrogen ions, high temperature, and a highly corrosive environment, it is also necessary for the separator to have a sufficiently high corrosion resistance.
To ensure the electrical conductivity and corrosion resistance of the separator, Japanese Patent Application Publication Nos. H11-162478 and H10-228914 disclose techniques of plating a noble metal such as gold on the surface of a metal separator.
Irrespective of improvement in corrosion resistance and electrical conductivity, such techniques as described above suffer from high manufacturing costs and generation of defects such as pin holes, which leads to low practicability.
Japanese Patent Application Publication Nos. 2003-276249 and 2003-272653 disclose techniques of plating a very thin film of gold on a metal separator for cost reduction. However, these techniques have problems in that there is a high possibility of corrosion related to pin holes caused by hydrogen generated during the gold coating, and that although gold is thinly coated thereon, it is still expensive compared to other wet coating.
PCT WO99/19927 discloses a technique in which relatively inexpensive carbon powder is dispersed on the surface of a metal separator, and spreads into a passive film of the separator through rolling and heat treatment to improve the electrical conductivity.
However, carbon spread into the passive film couples with chrome existing in a large amount in the passive film to precipitate chrome carbide, thereby reducing the content of chrome. As a result, the corrosion resistance on the surface of the material is deteriorated, providing an adverse influence on performance of the fuel cell.
Further, when such a fuel cell is applied to vehicles, the carbon powder is likely to be detached from the separator due to vibration during operation, and since stainless steel exhibits high contact resistance in the case of no proper pretreatment, it is not suitable to be used as a raw material for the metal separator.
Japanese Patent Application Publication No. 2000-353531 discloses a method that forms a titanium nitride film on the surface of the separator through high temperature nitridation of titanium. PCT WO2005/124913 A1 discloses a method, by which a thin Ti plate is formed with gas and coolant paths by pressing, and is subjected to sputtering and plasma nitriding treatment in a reducing gas atmosphere to form a nitrogen diffusion layer on the surface of the separator. However these prior techniques have difficulty with commercialization due to low productivity resulting from use of expensive Ti material and the vacuum process.
FIG. 1 is a view illustrating variation of an atomic ratio as a function of depth from the surface of a conventional stainless steel sheet that is commercially available.
The stainless steel sheet has a passive film that is formed from the surface of the stainless steel sheet to a depth of about 1.5 nm in an internal direction and contains great amounts of iron (Fe) and oxygen (O).
The passive film contains metallic components such as iron, nickel, chrome, etc., which will couple with oxygen in an outer environment to form an oxide film on the passive film.
As such, since iron oxides such as FeO, Fe2O3, and Fe3O4 formed on the surface of the stainless steel sheet by coupling of Fe and O cannot suppress corrosion of the separator in a sulfuric acid atmosphere at a high temperature of 80° C. and act as an insulator, the iron oxides lower the electrical conductivity of the separator, thereby causing deterioration in performance of the fuel cell.